Comparison of capture-recapture and visual count indices of prairie dog densities in black-footed ferret habitat

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1 Great Basin Naturalist Memoirs Volume 8 The Black-footed Ferret Article Comparison of capture-recapture and visual count indices of prairie dog densities in black-footed ferret habitat Kathleen A. Fagerstone U.S. Fish and Wildlife Service, Denver Wildlife Research Center, Bldg. 16, Denver Federal Center, P.O. Box 25266, Denver, Colorado Dean E. Biggins U.S. Fish and Wildlife Services, Denver Wildife Research Center, P.O. Box 916, Sheridan, Wyoming Follow this and additional works at: Recommended Citation Fagerstone, Kathleen A. and Biggins, Dean E. (1986) "Comparison of capture-recapture and visual count indices of prairie dog densities in black-footed ferret habitat," Great Basin Naturalist Memoirs: Vol. 8, Article 7. Available at: This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist Memoirs by an authorized editor of BYU ScholarsArchive. For more information, please contact scholarsarchive@byu.edu, ellen_amatangelo@byu.edu.

2 COMPARISON OF CAPTURE-RECAPTURE AND VISUAL COUNT INDICES OF PRAIRIE DOG DENSITIES IN BLACK-FOOTED FERRET HABITAT Kathleen A. Fagerstone' and Dean E. Biggins" Abstract. Black-footed ferrets {Mtistela nigripes) are dependent on prairie dogs (Cijnomys spp.) for food and on their burrows for shelter and rearing young. A stable prairie dog population may therefore be the most important factor determining the survival of ferrets. A rapid method of determining prairie dog density would be useful for assessing prairie dog density in colonies currently occupied by ferrets and for selecting prairie dog colonies in other areas for ferret translocation. This study showed that visual counts can provide a rapid density estimate. Visual counts of white-tailed prairie dogs {Cijnomys leucurus)were significantly correlated (r = 0.95) with mark-recapture population density estimates on two study areas near Meeteetse, Wyoming. Suggestions are given for use of visual counts. Recovery of the endangered black-footed ferret will involve the careful management of the only known population near Meeteetse, Wyoming, as well as captive breeding and translocation. Both ferret preservation and population recovery are dependent on the presence of prairie dog colonies. Ferrets have been most frequently observed in or near prairie dog colonies (Cahalane 1954, Henderson et al. 1969), and their original distribution probably corresponded closely to the range of the black-tailed (Cijnomys ludovicianus) and white-tailed prairie dogs (Hall 1981). The black-footed ferret relies on the prairie dog for approximately 90% of its diet (Henderson et al. 1969, T. M. Campbell personal communication) and on prairie dog burrows for shelter and rearing young. Prairie dog populations declined dramatically during the last century because of loss of habitat and poisoning. From an estimated 283 million ha occupied in the late 1800s (Merriam 1902), prairie dog colonies declined to less than 0.6 million ha by 1971 (Cain et al. 1971). The decline of the black-footed ferret during the last century is probably linked to the reduction in prairie dog populations. A model using growth rates of Siberian polecats to simulate those of black-footed ferrets estimated the annual prey requirement of the black-footed ferret to be 214 black-tailed prairie dogs (Stromberg et al. 1983). They assumed an intrinsic rate of growth of 1.5 for prairie dog populations and calculated the prairie dog population size required to support a ferret at 766. Because white-tailed prairie dogs are larger, their model predicted the annual prey requirement to be 186 animals and the required population size to be 666. In telemetric studies, a radio-tagged black-footed ferret preferred areas of dense prairie dog burrows within its home range (Biggins et al. 1985), and we postulate that high prairie dog densities are important to ferrets. A rapid method of determining prairie dog population density needs to be developed that can be used to assess the prairie dog populations at Meeteetse and that would allow us to monitor prairie dog populations at frequent intervals for potential problems, such as plague outbreaks or effects of oil development. A rapid density estimation procedure also could be used to assess prairie dog populations in colonies being considered for ferret translocation. Prairie dog population numbers have been estimated using a variety of methods. Markrecapture is a reliable method for estimating the density of prairie dogs because these animals have relatively small home ranges and are readily trapped. However, mark-recapture is labor intensive and can be done only on relatively few plots; it is therefore impractical for estimating animal density over large areas. Closing burrows and counting the number 'U.S. Fish and Wildlife Service, Denver Wildlife Research Center, Bldg. 16, Denver Federal Center, P.O. Box , Dei ^U.S. Fish and Wildlife Service, Denver Wildlife Research Center. P.O. Box 916, Sheridan, Wyoming Reference to trade names does not imply endorsement by the federal government. 94

3 1986 Fagerstone, Biggins: Density 95 reopened after 1 or 2 days is a method frequently used in conjunction with control programs, where pretreatment and posttreatment counts are compared to determine the effectiveness of rodenticide applications to prairie dog populations (Tietjen 1976). The method provides an index to prairie dog activity that may have little correlation with actual population trends (Knowles 1982); results can be variable with this technique because one prairie dog can reopen more than one burrow. Visual counts may provide a quick method of measuring prairie dog density; prairie dogs are well suited for visual counts because of their large size, their diurnal activity patterns, and their tendency to live in social colonies. Visual counts were used by Knowles (1982) to estimate black-tailed prairie dog numbers, but their precision was not assessed for whitetailed prairie dogs. This study evaluated the use of visual counts to monitor white-tailed prairie dog densities by comparing visual counts with mark-recapture data. Study Area The study was conducted 30 km southwest of Meeteetse, in Park County, Wyoming. White-tailed prairie dogs occur in colonies on about 3000 ha (Clark et al. 1984) throughout this area. We studied two colonies located between 2280 and 2380 m in elevation on short- to midgrass rangeland. Methods Mark-Recapture Prairie dog populations were censused by mark-recapture during May and July 1984 and May A 360 x 360 m trapping grid was established on each of the two study colonies using 169 National^ live traps (48 x 15 x 15 cm) located at 30 m intervals. The grid was subdivided into nine 120 x 120 m subplots. Before each trapping period, the traps were wired open and baited with flaked oats for a two-day familiarization period. During the subsequent five-day trapping period, the traps were baited with oats and checked during the morning; they were closed at midday to avoid prairie dog mortality caused by heat stress. The trapped prairie dogs were aged (juvenile or adult), sexed, ear-tagged with monel No. 1 fingerling fish tags, and released at the point of capture. Population estimates for each of the trapping periods were computed using the computer program CAPTURE (White et al. 1978). Otis et al. (1978) have provided a detailed reference on the theory behind program CAPTURE. Visual Counts Prairie dogs on the study area were observed prior to the initiation of this study. They exhibited a bimodal activity pattern with peak numbers aboveground between 0700 and 1000 hours and with a second but lower peak between 1500 and 1800 hours. This bimodal activity pattern is similar to that observed by Tileston and Lechleitner (1966) and Clark (1977) for white-tailed prairie dogs and by Althen (1975) for black-tailed prairie dogs. Visual counts were therefore conducted during the peak activity period in the morning on four consecutive days following the trapping period. During May 1984 prairie dogs were counted from portable 3-m-high towers erected in the center of each 120 x 120 m subplot. Counts from the center of each subplot proved labor intensive, so during July 1984 and May 1985 prairie dogs were counted from two locations outside the entire 360 x 360 m grid; observers were located a minimum of 30 m from the grid to minimize disturbance of animals. Two observers counted the grid fi-om each location. Prairie dogs on each 120 x 120 m plot were counted during a four-minute period by scanning the plot with binoculars and a 15X spotting scope. Three counts were made daily of each plot during a two or threehour period. Plots were counted in the same sequence and at synchronized times by observers at both locations. Prairie dogs that were located on the borders between two plots were counted if they were on the north and east edges and not counted if on the south and west edges. Statistics Simple linear correlation coefficients were computed (1) between the highest total count of individual prairie dogs over the entire 360 x 360 m grid and the population density generated by program CAPTURE for the corresponding five-day period and (2) between the highest single count of individual prairie dogs

4 96 Great Basin Naturalist Memoirs No. 8!= 60 S MAXIMUM VISUAL COUNTS Fig. 1. Prairie dog population estimates on 360 by 360 m grids (x-axis) plotted against maximum visual counts on the same areas (y-axis). The simple linear regression equation is: y = x. per 120 X 120 m plot and the number of prairie dogs trapped on that plot during the corresponding five-day trapping period (insufficient numbers ofprairie dogs were trapped on each 120 x 120 m plot to generate a satisfactory population density). The variation associated with location, observer, day, and trial (three counts per day) was determined using a procedure on SAS (SAS 1985) that estimates variance components (PROG VARGOMP). Results There was a high correlation between the population densities estimated by GAPTURE and the highest number of animals counted visually across the entire 360 x 360 m grid during the corresponding period (r = 0.95, P = 0.004, Fig. 1). The simple linear regression equation is: y = x, where y is the maximum visual count and x is the population density. Population density correlated better with visual counts than the total number of animals trapped (r = 0.84). Also, the maximum number counted provided a better correlation than the average of a series of counts (r=0.74). There was a lower correlation between the highest count and number trapped per 120 x 120 m sub-plot (r = 0.69); when analyzed separately by time period the correlation was highest during May 1984 (r = 0.86) and lower during July 1984 and May 1985 (r = 0.70 and 0.61, respectively). Visual counts on small areas may therefore not be as representative of actual densities as counts on larger areas. Variance component estimation revealed that trials (counts per day) accounted for the most variation in the data (Table 1). This was expected because counts were begun in the morning as prairie dogs emerged from burrows and were continued until prairie dogs became less active above ground in midmorning. During any day, counts were normally low at first, increased to the maximum count, then decreased. Location accounted for a large portion of the variation in the data on area 1 but only a small portion of the variation on area 2. Location was important on study area 1 because tall grass grew on a portion of the study area between the time the area was chosen and the time when visual counts were begun. The grass made counting prairie dogs on part of the plot difficult from one of the two locations. When trials were removed from the analysis and only the maximum count by each observer per day was used, location still accounted for a large portion of the variation in the data on area 1. Day variation was small for area 1 (only one-third of location variation) but was comparatively large for area 2. Observer variation was negligible, but a large variance component existed for observer-day interaction. This would indicate that variability was present between observers over the four-day period but that observers had no consistent bias toward high or low counts. Discussion Visual counts appear to provide a useful index to prairie dog population densities that can be used to monitor prairie dog populations at Meeteetse and to assess ferret relocation sites. Mark-recapture is a reliable

5 1986 Facerstone, Biggins. Density 97 Table 1. Components of variance for prairie dog visual counts of two study areas near Meeteetse, Wyoming. On each study area, counts were conducted over a four-day period from two locations by two obser\'ers at each location. The magnitude of the variance indicates the relative influence of each item in the model to the overall variation.

6 AND R 98 Great Basin Naturalist Memoirs No. 8 teraction and because a large day component was present on one area, we recommend that counts be made over several days by the same observer. Although visual counts can be a precise method of estimating prairie dog populations, they should be used with caution. Precision is based upon their repeatability. Therefore, the observer, location, and time of day should remain constant between one count and the next whenever possible. The area to be counted should be predetermined and its boundaries well marked so that prairie dogs outside the area will not be counted. Systematic scans of an area for predetermined time periods can minimize the possibility of counting animals more than once; the only animals counted twice are those that move across the study area during the scan. If conducted following the guidelines suggested, visual counts can be a valuable technique for estimating prairie dog densities. Acknowledgments We greatly appreciate the statistical advice of R. Engeman (Denver Wildlife Center statistician). G. H. Matschke and P. L. Hegdal kindly reviewed the manuscript. Literature Cited Althen, C L The interaction of circadian rhythm and thermal stress in controuing activity in the black-tailed prairie dog. Unpublished dissertation, University of Colorado, Boulder. 168 pp. Biggins, D. E., M Schroeder, S Forrest, and L Richardson Movements and habitat relationships of radio-tagged black-footed ferrets. Pages in S. Anderson and D. Inkley, eds., Black-footed Ferret Workshop Proc, Laramie, Wyoming, September 18-19, Wyoming Game and Fish Publ., Cheyenne. Cahalane, V H Status of the black-footed ferret. J. Mammal. 35: CainS A,J A Kadlec. D L Allen.R A Cooley, M.G. HoRNOCKER, A S. Leopold, and F. H. Wagner Predator control Report to the Council on Environmental Quality and the Department of the Interior by the Advisory Committee on Predator Control. Inst, for Environ. Quality, University of Michigan, Ann Arbor. 207 pp. Clark, T W Ecology and ethology of the whitetailed prairie dog (Ci/no/rii/.s/ewcurus). Publ. Biology and Geology No. 3, Milwaukee Public Museum. 97 pp. Clark, T W, L Richardson, D Casey, T M Campbell, III, ands C Forrest Seasonality of blackfooted ferret diggings and prairie dog burrow plugging. J. Wildl. Manage. 48: Davis, A H Winter activity of the black-tailed prairie dog in north-central Colorado. Unpublished thesis, Colorado State University, Fort Collins, 45 pp. Hall, E R The mammals of North America, 2d ed. Ronald Press, New York pp. Henderson, F. R., P F. Springer, and R. Adrian The black-footed ferret in South Dakota. South Dakota Dept, Game, Fish and Parks Bull. 37 pp. Knowles, C J Habitat affinity, populations, and control of black-tailed prairie dogs on the Charles M. Russell National Wildlife Refuge. Unpublished dissertation. University of Montana, Missoula. 171 pp. Merriam, C H The prairie dog of the Great Plains. Pages in L. C. Everard, ed.. Yearbook U.S. Dept. Agric, Washington, D.C, Otis, D L, K P Burnham, G C White, and D R Anderson 1978, Statistical inference from capture data on closed animal populations. Wildl. Monogr. 62:1-135, SAS SAS Users guide: statistics, version 5 edition. SAS Institute Inc., Cary, North Carolina. 956 pp. Stromberg, M R, L Rayburn, andt W. Clark Black-footed ferret prey requirements: an energy balance estimate, J. Wildl. Manage. 47:67-73, TiETjEN, H P 1976, Zinc phosphide its development as a control agent for black-tailed prairie dogs. USDl, Fish and Wildl, Ser, Spec. Sci. Rept. Wildl. No pp. TiLESTON, R R Lechleitner 1966, Some comparisons J V, of the black-tailed and white-tailed prairie dogs in north-central Colorado. Amer. Midi. Nat, 75: , White, G C, K P Burnham, D. L. Otis, and D. R. Anderson, 1978, User's manual for program CAP- TURE, Utah State University Press, Logan, Utah. 40 pp.